1. Field of the Invention
The present invention relates to a semiconductor device and more particularly a semiconductor device including a power device and a driver which drives the power device.
2. Description of the Related Art
Power devices are used in a power electronics based apparatus such as a universal inverter and an uninterruptible power supply (UPS). Insulated gate bipolar transistors (hereinafter, referred to as “IGBT”) are becoming mainstream as the power devices. Further, an intelligent power module (hereinafter, referred to as “IPM”) in which an IGBT, its driver, and its protector are integrated into one module has been recently put to practical use. The driver in the IPM is required to perform the ON-OFF control swiftly and securely.
A driver for the IGBT in a general IPM will be described in the following. The driver switches on and off the IGBT by controlling the gate voltage of the IGBT. The driver controls the gate voltage in correspondence to a control signal input to the driver from a controller outside of the IPM. The transmission of the control signal from the controller to the driver is usually done by using a photo-coupler, so that the driver is connected to the photo-coupler at its input side. The detail of the input-side configuration of the driver will be described in the following. The driver includes a capacitor, a comparator, and a drive controller. One end of the capacitor is connected to one input terminal (a first input terminal) of the comparator, and the other end thereof is grounded. An output terminal of the comparator is connected to an input terminal of the drive controller. An output terminal of the drive controller is connected to a gate terminal of the IGBT.
The capacitor, which eliminates outside noise, is connected to the photo-coupler. The terminal of the capacitor, which is connected to the first input terminal of the comparator, is also connected to a collector terminal of a phototransistor of the photo-coupler. An emitter terminal of the phototransistor is grounded, and a collector terminal of the phototransistor is connected to a power supply through a resistor. In this configuration, when the control signal is input to the primary side of the photo-coupler from the controller, the control signal is transmitted to the secondary side (corresponding to the phototransistor) of the photo-coupler, so that the phototransistor switches on or off. When the phototransistor switches off, current flows into the capacitor from the power supply through the resistor, that is, the capacitor is charged. While, when the phototransistor switches on, current flows into the phototransistor from the power supply through the resistor. Here, if the capacitor has been charged, current flows from the capacitor, and the capacitor is discharged. In this way, the capacitor is charged or discharged in correspondence to the control signal from the controller, so that the voltage applied to the first input terminal of the comparator is varied.
The comparator compares the value of the voltage input to the first input terminal thereof with that of the voltage (a fixed reference voltage) input to the other terminal thereof (referred to as “a second input terminal”), and outputs a voltage signal into the drive controller based on the comparison results. Specifically, the comparator outputs a high-level (H) signal, if the voltage input to the first input terminal thereof is higher than the fixed reference voltage, and outputs a low-level (L) signal, if the voltage input to the first input terminal thereof is equal to or lower than that. The drive controller outputs a voltage signal, which is in correspondence to the output voltage of the comparator, into the IGBT. The IGBT switches on and off in correspondence to the output signal of the drive controller. Generally, the IGBT switches off when the output signal of the comparator is at high level, and the IGBT switches on when the output signal of the comparator is at low level.
In the above circuit, the resistance of the resistor connected to the collector terminal of the phototransistor is set relatively high in order to limit the current flowing into the photo-coupler within the current capacity. Here, the charging time for the capacitor is represented by the product of the resistance R and the capacitance C, i.e. the time constant CR. Therefore, if the resistance is large, i.e. the impedance is high, then the capacitor is charged slowly. When the capacitor is charged, the IGBT is turned off. If the capacitor is charged slowly, the rise speed of the voltage at a signal line (referred to as “a voltage-input signal line”), connected to the first input terminal of the comparator, becomes low. That is, the voltage at the voltage-input signal line stays near the threshold value, by which the IGBT is switched off, for a time before and after the voltage at the voltage-input signal line exceeds the threshold value. If, at this time, a large amount of dV/dt (this indicates the rate of change of the voltage between the collector and emitter terminals of the IGBT, i.e. so-called spike voltage) occurs, the voltage at the voltage-input signal line can fluctuate by, for example, the electromagnetic induction between wiring lines. If the fluctuation of the voltage occurs, it causes serious malfunction of the driver and the IGBT, because the fluctuation occurs near the threshold value.
In order to prevent the above-mentioned malfunction due to a large amount of dV/dt, one of conventional drivers includes a drive circuit which drives a gate of a power device, a protection circuit which protects the power device, a control circuit which outputs a signal that switches the power device on and off to the drive circuit in correspondence to a signal from the outside and a signal from the protector, and a regulator circuit which generates a voltage lower than the voltage of the external control power supply to provide it to the protection circuit and the control circuit (For example, refer to the Japanese Patent laid-open No. 5-276000). According to this driver, even if the voltage of the control power supply fluctuates by dV/dt in switching of the power device, the fluctuation is absorbed in the regulator circuit, so that the malfunctions of the control circuit and the protection circuit can be prevented.
There has also been a configuration in which, a pull-up circuit consisted of a depression-type metal-oxide semiconductor (hereinafter, referred to “MOS”) with its gate and source terminals connected to each other and a p-channel MOS, a pull-down circuit consisted of a depression-type MOS with its gate and source terminals connected to each other and an n-channel MOS, a Zener diode, and a capacitor are arranged between a resistor and a photo-coupler, and driver (For example, refer to Laid-Open Japanese Patent Publication 2002-300019.). In this configuration, when the photo-coupler is off, the voltage of the capacitor connected to the photo-coupler is at high level (pull-up), so that a large current flows into the driver through the pull-up circuit. While, when the photo-coupler is on, the voltage of the capacitor is at low level (pull-down), so that a current flows through the pull-down circuit. According to this configuration, when the photo-coupler is off, the malfunction due to noise can be prevented because a large current flows into the driver. Further, when the photo-coupler is on, power consumption can be reduced because the current bypassing to the phototransistor is small.
Further, there has been a configuration in which two drive means having different power supplies are connected to a gate terminal of a power device, and the drive speed of the power device is varied by switching between the two driver means (For example, Laid-Open Japanese Patent Publication No. 10-23743.) According to this configuration, during a first period when a surge voltage may not occur, the power device is driven at high speed so that the switching loss can be reduced, and during a second period when a surge voltage may occur, the power device is driven at low speed so that the surge voltage can be restrained.
The drivers disclosed in such as the Laid-Open Japanese Patent Publication No. 10-23743 are aimed at reducing a switching loss during switching of the power device and restraining a surge voltage. In such publications, the malfunction due to above-described dV/dt isn't mentioned at all.
The drivers disclosed in Laid-Open Japanese Patent Publications No. 5-276000 and No. 2002-300019 have configurations to prevent the malfunction due to dV/dt occurred in switching the power device. However, a driver in which the malfunction can be prevented more precisely, i.e. accurately at the time of necessity, is desired.